One prior art approach to the problem of signal fading is to operate a transmitter portion of a communication link at full power consumption to deliver the required linearity of the received system signal under almost any circumstances. When conditions exist that fading of the signal occurs, the power received at the receiving end of the communication link will drop. When the error rate, quality or integrity of the received signal becomes low enough, a service channel in the communications system will alarm the transmitter to either switch to a standby power amplifier, or to change signal frequency channels in the hope that other frequencies are not as affected by the conditions causing the fading. Otherwise, there will be a loss of communication traffic.
Another prior art approach has been to provide an automatic power control, whereby either different amplifier circuits are switched in and out of operation between normal and fading conditions instead of switching to a standby power amplifier or to change signal frequency channels.
The present invention changes the bias level voltage to one or more of the output amplification stages in the transmitter power amplifier such that the output stages are operated in the less linear region (and at accordingly lower power consumption rate) under normal conditions. Under such normal conditions, a receiver can tolerate a lower quality signal, because there is adequate power being received. Then, when fading is detected, the transmitter is requested to increase the output power. In the present system, this increase in power is obtained by returning the bias of the output stages to a more linear amplification area. A more distortion free signal obtained at normal high power levels overcomes the effects of fading and if fading continues, the signal can be increased even more. The signal then becomes more non-linear again to get even more power out.
The present concept can be applied to raise the bias voltage above the normal design level whereby if fading continues, the signal power output can be increased even more. At this increased power level, the linearity of the signal again decreases, but under some operational conditions, the non-linearity may be less important than the desirability of an increased amount of output power.
The operation of the amplifiers at lower RF power levels during normal conditions creates less channel interference, and even further improves system operation over the prior art approach used by the present assignee.
It is accordingly an object of the present invention to provide an improved automatic power control system.